I feel like you are being intentionally dense. Show me some examples of customized KDE builds, at the source level. You don’t even have to look to find 100s probably 1000s of gnome extensions. They are not the same.
First, you should subtract from those "100s probably 1000s" what's possible via the identical KDE mechanism that you were aware of in the other comment ("some light scripting or create your own plugins").
Then once you get to a less-obviously-wrong number you should think about the all the vibe coding that resulted in nothing useful, also a very common pastime nowadays, and not count those towards achieving the goal of "easiest DE to customize"
Think think about whether there might be other factors besides "ease" that could lead to the difference. Like, maybe the base KDE is already more customizable, so you wouldn't need to vibe code any custom builds to achieve the same.
Basically, just take to heart your own
> They are not the same
...only dig just a bit deeper